1. Field of the Invention
This invention relates generally to treatment of subterranean formations and, more specifically, to chemical treatment of subterranean hydrocarbon-bearing formations to inhibit the adsorption of asphaltenes onto formation surfaces. In particular, this invention relates to treatment of subterranean hydrocarbon-bearing formations with sacrificial compounds, such as lignosulfonates, to inhibit the adsorption of asphaltenes and similar compounds onto formation surfaces.
2. Description of Related Art
Asphaltene deposition is a common problem in oil production operations. Asphaltenes and related materials may deposit from crude oil in surface production equipment, wellbore tubulars, and/or on subterranean formation surfaces. Besides resulting in production downtime and necessitating potentially expensive remedial and/or stimulation treatments, asphaltene deposition may also result in reservoir wettability changes when deposited in the formation matrix. Reservoir wettability is a significant controlling factor in oil recovery efficiency because of its influence on location, distribution, and flow characteristics of reservoir fluids. Consequently, changes in wettability may impact recovery efficiency by affecting capillary pressure, relative permeability and residual fluid saturations. Deposition of asphaltenes from crude oil is widely believed to alter reservoir wettability toward an oil-wet condition. Because oil recovery efficiency for a given reservoir typically declines as the reservoir becomes more oil-wet in character, deposition of asphaltenes may have an adverse impact on ultimate oil recovery.
Asphaltenes and related compounds are polar components that may be present in crude oil. These polar components may also include maltenes, resins and/or polar aromatic compounds. These compounds are naturally occurring components of crude oil and may exist as a stable colloidal suspension or dispersion in oil. The colloids may be composed of micellar structures consisting of asphaltenes and maltenes. Asphaltenes may include, among other things, aromatic and alkyl groups in combination with heteroatoms, such as nitrogen, sulfur or oxygen. Colloid and micelle destabilization with subsequent aggregation, adsorption, and bulk precipitation of polar crude oil components in a reservoir may be caused by chemical, electrical or physical stimulation. Such stimuli may include exposure to foreign fluids, changes in temperature and pressure, changes in reservoir fluid composition and flow through a permeable reservoir matrix. These stimuli may be introduced to a reservoir during drilling, completion and workover processes, or may occur during production of formation fluids through the matrix to the wellbore.
In the past, asphaltene deposition on formation surfaces has typically been treated with aromatic solvents, such as xylene or toluene. However, these solvent treatments typically only succeed in partial removal of deposited asphaltenes. In addition, these treatments may be expensive due to large volumes required, and typically need to be repeated as asphaltenes are redeposited over time. In addition, asphaltenes solubilized by aromatic solvents may reprecipitate elsewhere in the formation.
Other types of treatments have been directed at preventing asphaltene deposition. These treatments generally focus on stabilizing crude oil chemistry by injecting a "batch" of chemicals into a formation. To prevent or inhibit deposition of asphaltenes, these treatments rely on those chemicals to mix with the crude oil at very low concentrations as the crude oil is being produced. In this way, these treatments attempt to chemically stabilize asphaltene components in the crude oil through mixing with the crude oil as the oil is being produced. The primary failure mechanism for these treatments is that the injected inhibitor chemicals can flow out of the reservoir as easily as the crude oil so the volume of oil treated is very small which results in a very short term inhibition effect. These same chemicals have also been applied by continuous injection down a well annulus or a chemical injection tubing string in a wellbore. The injection string conveys the chemical to the bottom of the well and causes the inhibitor chemicals to mix with the crude oil at that location. Although deposition of asphaltic components in the wellbore may be prevented, this type of treatment does not address the prevention of asphaltene deposition in the reservoir matrix.